The real question, in my mind, about overheating or overpricing, is what the alternatives are.
For the average hands-off "investor", there really aren't any alternatives to the bond and equities markets, other than a bank account or something like short-term deposits, and physical metals.
I talk about real estate, art, businesses, etc., but realistically, for the average person these are not options.
Sure there is wiggle room within the markets, but that money remains within the markets.
Ultimately, the markets are still "worth it" to everybody, even though some perceive them as overpriced or overheated, until they find a better, PALATABLE AND MANAGEABLE alternative.
Let's face it, there isn't one. "Investing" in the markets is insanely easy, and purposefully so--for instance, compare owning REIT's to owning actual real estate...REIT's are comparatively very easy, and incur very low transactional costs.
So I take the "overheating" and "overpricing" talk with a grain of salt, in general. For me personally, they are overpriced, and I put my money to work elsewhere--however, in general, I don't think they are necessarily overpriced, in the sense that there will be a big correction based on that overpricing. I still believe there will be a downturn or even a crash, but I don't think it will be precipitated by "overpricing" concerns.